Methods and systems for reducing noise in optoelectronic oscillators

ABSTRACT

Optoelectronic oscillator systems and an optoelectronic oscillator noise reduction method. One example of an optoelectronic oscillator system includes an optical source positioned at a first end of a fiber-optic path, the optical source being configured to transmit an optical signal along the fiber-optic path, an optical modulator positioned to receive and modulate the optical signal based on at least a reference signal, a retro-reflector positioned at a second end of the fiber-optic path, the retro-reflector being configured to receive and retro-reflect the optical signal, the retro-reflected optical signal having at least a frequency range of inherent fiber noise canceled, and an optical circulator positioned along the fiber-optic path between the optical modulator and the retro-reflector, the optical circulator being configured to direct the optical signal to the retro-reflector and direct the retro-reflected optical signal along a feedback path to a first photodetector to generate the reference signal.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 62/103,691, titled “METHODS ANDAPPARATUS FOR REDUCING NOISE IN OPTOELECTRONIC OSCILLATORS,” filed onJan. 15, 2015, which is hereby incorporated herein by reference in itsentirety.

BACKGROUND

Optoelectronic oscillators (OEOs) are high-frequency radio frequency(RF) sources with very low phase noise. Unfortunately, conventionalarchitectures still suffer from some phase noise, which reduces theoverall spectral purity of the source. The two basic sources of noise inoptoelectronic oscillators include fiber dependent amplitude noise fromRayleigh and Brilloin scattering, which is converted to phase noiseduring photo-detection processes, and phase noise induced fromenvironmental conditions. Several and ongoing attempts have been made toeliminate these sources of noise in optoelectronic oscillators. Theseinclude, for example, optical and electronic filtering and laserfrequency modulation. Additional attempts to reduce noise have includedapproaches for suppressing amplitude to phase conversions duringphoto-detection processes.

SUMMARY OF THE INVENTION

Aspects and embodiments are directed to systems for reducing noise inoptoelectronic oscillators, and methods of using the same. Certainexamples involve the implementation of a double pass configurationoptoelectronic oscillator system, which may include optical circulators,retro-reflectors, couplers, or other similarly functioning elements.These examples allow for inherent fiber noise cancellation throughactive, or passive, approaches. Additional components and devices, suchas amplifiers, modulators, phase shifters, and optical filters, may beincluded to further reduce inherent fiber noise and allow an increasedfiber-optic path length while maintaining a low close-in phase noise.

At least one aspect is directed to an optoelectronic oscillator system.In one embodiment, the optoelectronic oscillator system includes anoptical source positioned at a first end of a fiber-optic path, theoptical source being configured to transmit an optical signal along thefiber-optic path, an optical modulator positioned to receive the opticalsignal from the optical source and modulate the optical signal based onat least a received reference signal, a retro-reflector positioned at asecond end of the fiber-optic path, the retro-reflector being configuredto receive and retro-reflect the optical signal along the fiber-opticpath, the retro-reflected optical signal having at least a frequencyrange of inherent fiber noise canceled, and an optical circulatorpositioned along the fiber-optic path and interposed between the opticalmodulator and the retro-reflector, the optical circulator beingconfigured to direct the optical signal to the retro-reflector anddirect the retro-reflected optical signal along a feedback path to afirst photodetector to generate the reference signal.

According to an embodiment, the optoelectronic oscillator system furtherincludes an optical component positioned along the fiber-optic path andinterposed between the retro-reflector and the optical circulator, andan electrical component positioned along the feedback path andinterposed between the first photodetector and the optical modulator,and at least one of the optical component and the electrical componentincludes one of an amplifier, a filter, an additional modulator, and aphase shifter. In a further embodiment, the optoelectronic oscillatorsystem further includes a second photodetector interposed between theoptical modulator and the electrical component. In one embodiment, theelectrical component is configured to receive an output from the secondphotodetector, receive the reference signal from the firstphotodetector, and transmit a control signal to the optical component tocancel the frequency range of inherent fiber noise in theretro-reflected optical signal.

In one embodiment, the optoelectronic oscillator system further includesa fiber link delay positioned along the fiber-optic path and interposedbetween the retro-reflector and the optical component, and the fiberlink delay has a length of substantially 10 km. In an embodiment, thefiber link delay includes a single optical fiber link.

According to one embodiment, the retro-reflector is configured cancelthe frequency range of inherent fiber noise by retro-reflecting theoptical signal. In an embodiment, the retro-reflector includes a phaseconjugate mirror.

According to an aspect, provided is an optoelectronic oscillator system.In one embodiment, the optoelectronic oscillator system includes a firstoptical source positioned at a first end of a fiber-optic path andoptically coupled to a first optical modulator, the first optical sourcebeing configured to transmit a first optical signal along thefiber-optic path, a second optical source positioned at a second end ofthe fiber-optic path and optically coupled to a second opticalmodulator, the second optical source being configured to transmit asecond optical signal along the fiber-optic path, and the secondmodulator being configured to cancel at least a first frequency range ofnoise in the second optical signal, a fiber link delay positioned alongthe fiber-optic path and interposed between the first optical modulatorand the second optical modulator, and a first optical circulatorpositioned along the fiber-optic path and interposed between the firstoptical modulator and the fiber link delay, the first optical circulatorbeing configured to receive the second optical signal from the secondoptical modulator and direct the second optical signal along a firstfeedback path to a first photodetector to generate a first referencesignal.

In one embodiment, optoelectronic oscillator system further includes asecond optical circulator positioned along the fiber-optic path andinterposed between the second optical modulator and the fiber linkdelay, the second optical circulator being configured to receive thefirst optical signal from the first optical modulator and the fiber linkdelay and direct the first optical signal along a second feedback pathto a second photodetector to provide a second reference signal.

According to an embodiment, the optoelectronic oscillator system furtherincludes a first optical component positioned along the fiber-optic pathand interposed between the first optical circulator and the fiber linkdelay, and a second optical component positioned along the fiber-opticpath and interposed between the second optical circulator and the fiberlink delay. In one embodiment, the optoelectronic oscillator systemfurther includes a first electrical component positioned along the firstfeedback path and interposed between the first photodetector and thefirst optical modulator, and a second electrical component positionedalong the second feedback path and interposed between the secondphotodetector and the second optical modulator, and at least one of thefirst electrical component, the second electrical component, the firstoptical component, and the second optical component, includes one of anamplifier, a filter, an additional modulator, and a phase shifter.

According to one embodiment, the optoelectronic oscillator systemfurther includes a third photodetector interposed between the firstoptical modulator and the first electrical component, and a fourthphotodetector interposed between the second optical modulator and thesecond electrical component. In an embodiment, the first electricalcomponent is configured to receive an output from the thirdphotodetector, receive the first reference signal from the firstphotodetector, and transmit a control signal to the first opticalcirculator to cancel at least a second frequency range of the noise inthe second optical signal. In one embodiment, the fiber link delayincludes a single optical fiber link having a length of substantially 10km. According to an embodiment, the second optical modulator isconfigured to receive the second reference signal and amplitude modulatethe second optical signal.

According to another aspect, provided is an optoelectronic oscillatornoise reduction method. In one embodiment, the method includestransmitting an optical signal from an optical source positioned at afirst end of a fiber-optic path along the fiber-optic path, modulatingthe optical signal with an optical modulator positioned to receive theoptical signal along the fiber-optic path based on at least a referencesignal, receiving and retro-reflecting the optical signal at aretro-reflector positioned at a second end of the fiber-optic path, theretro-reflected optical signal having at least a frequency range ofinherent fiber noise canceled, and directing, with an optical circulatorpositioned along the fiber-optic path, the retro-reflected opticalsignal along a feedback path to a photodetector to generate thereference signal.

According to an embodiment, retro-reflecting the optical signal includescanceling the frequency range of inherent fiber noise in theretro-reflected optical signal. In an embodiment, the retro-reflectorincludes a phase conjugate mirror, and retro-reflecting the opticalsignal includes reversing a propagation direction of the optical signal.

In an embodiment, the method further includes receiving the opticalsignal at a second photodetector interposed between the opticalmodulator and an electrical component positioned in the feedback path,receiving the reference signal and an output of the second photodetectorat the electrical component, and transmitting a control signal from theelectrical component to an optical component positioned along thefiber-optic path to cancel the frequency range of inherent fiber noisein the retro-reflected optical signal.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is an example graphical illustration of the fiber inducedresidual phase noise in a 10 GHz carrier wave corresponding to a 4 kmlength of optical fiber;

FIG. 2 is an example schematic architecture for an optoelectronicoscillator system with reduced phase noise, according to aspects of thepresent invention;

FIG. 3 is an example schematic architecture for an alternately coupledoptoelectronic oscillator system, according to aspects of the presentinvention; and

FIG. 4 is an example process flow for an optoelectronic oscillator noisereduction method, according to aspects of the present invention.

DETAILED DESCRIPTION

Aspects and embodiments are directed to an approach for reducing phasenoise in optoelectronic oscillators. Certain examples involve theimplementation of a double pass configuration optoelectronic oscillatorsystem, which may include optical circulators, retro-reflectors,couplers, or other similarly functioning elements, positioned within afiber-optic path of the system. These examples allow for inherent fibernoise cancellation through active, or passive, approaches. Additionalcomponents and devices, such as amplifiers, modulators, phase shifters,and optical filters, may be included to further reduce inherent fibernoise (and/or environmentally induced noise) and allow an increasedfiber-optic path length, with a low close-in phase noise.

Oscillators with a high stability timing and low close-in phase noiseare desired for numerous applications. Several devices and systems havebeen proposed for generating a reference oscillator with thesecharacteristics. One such device includes an optoelectronic oscillator.Generally, an optoelectronic oscillator is an optical-electrical hybriddevice that generates spectrally pure microwave and millimeter wavesignals. Specifically, optoelectronic oscillators may include a laserand an optical modulator coupled by a fiber-optic delay line to aphotodetector.

While increasing the length of the fiber-optic delay line inconventional optoelectronic oscillators has been generally understood toincrease the performance of the device by reducing the close-in phasenoise, current research has shown that phase noise grows as the fiberlength is increased (See, for example, Docherty et al.,“Rayleigh-Scattering-Induced RIN and Amplitude-to-Phase Conversion as aSource of Length-Dependent Phase Noise in OEOs”, IEEE Photonics Journal,Vol. 5, No. 2, April 2013, Pages 1-15). Specifically, Rayleigh andBrilloin scattering limits the performance of optoelectronic oscillatorsas fiber length is increased. For example, FIG. 1 illustrates an exampleof the increase in fiber induced phase noise in a 10 GHz carrier wave asthe length of the optical fiber increases. A first trace 102 representsthe base line phase noise of a low-phase noise source, and a secondtrace 104 represents the residual phase noise of the source afterpropagating through an optical fiber having a length of 4 km. As shown,increasing the length of the optical fiber by 4 km increases the phasenoise by more than 20 dB for frequencies less than 10 kHz (i.e., thedifference between the baseline noise and the phase noise represented bythe second trace 104).

Double pass strategies have shown significant ability to reduce noisecharacteristics in optical-frequency transfer experiments (See, forexample, DROSTE et al., “Optical-Frequency Transfer over a Single-Span1840 km Fiber Link”, American Physical Society, Physical Review Letters,Sep. 13, 2013, Pages 110801-1 to 110801-5). Aspects and embodimentsdiscussed herein include an approach related to experiments tested onfiber-optic links of much greater length. In particular, aspects andembodiments discussed herein permit a larger bandwidth of phase noisecancellation than previously tested approaches (e.g., due to reducedfiber lengths), and in particular, permit the independent and individualcorrection and cancelation of effects from inherent fiber noise andenvironmentally induced fiber length fluctuations. As discussed herein,inherent fiber noise includes at least optical fiber-length-dependentflicker noise, such as noise from impurities in the optical fiber.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Also,the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.Any references to front and back, left and right, top and bottom, upperand lower, and vertical and horizontal are intended for convenience ofdescription, not to limit the present systems and methods or theircomponents to any one positional or spatial orientation.

Some examples of architectures for an optoelectronic oscillator systemwith reduced phase noise are shown in FIG. 2 and FIG. 3. It is to beappreciated that these are not the only possible architectures, and thatother components may be added and/or components may be removed tooptimize performance. Turning to FIG. 2, the optoelectronic oscillatorsystem 200 may include an optical source 202, an optical modulator 204,a retro-reflector 210, and an optical circulator 206, positioned along afiber-optic path 222. As discussed herein, the optical system 200 mayinclude an optical fiber extending the length of the fiber-optic path222. The optical fiber may optically couple each component discussedherein as being positioned along the fiber-optic path 222. The system200 may also include additional components, such as a firstphotodetector 212, a second photodetector 224, a fiber link delay 208,one or more optical component, one or more electrical component, and oneor more interconnections between the various components of the system200. In FIG. 2, the component blocks 214, 216 labeled “misc.” refer tothe one or more electrical component and one or more optical component,which may include a filter, combiner, amplifier, additional modulator,etc. In particular, the component block 214 refers to the one or moreelectrical component and the component block 216 refers to the one ormore optical component.

According to certain embodiments, by utilizing one or more round-tripsthrough the same fiber-optic path, active or passive techniques can beused to reduce the resultant phase noise and improve the performance ofthe optoelectronic oscillator system 200. Such techniques may be usedalone, or as part of another combined approach to improve signalquality. In one example, the optical source 202 is positioned at a firstend of the fiber-optic path 222, and configured to transmit an opticalsignal along the fiber-optic path 222 and in a direction of the opticalmodulator 204. The optical source 202 may include a fiber-coupleddistributed feedback (DFB) laser, or a solid-state laser. As shown, theoptical source 202 is in optical communication with the opticalmodulator 202. While in one embodiment, the optical source 202 maygenerate an optical signal having a wavelength within the spectrum ofvisible light, in various other embodiments, the optical source 202 maygenerate an optical signal of any other suitable wavelength ofelectromagnetic radiation. For example, in certain embodiments theoptical source 202 may generate a near infrared (NIR) optical signal.

As FIG. 2 shows, the optical modulator 204 is positioned along thefiber-optic path 222 and aligned to receive the optical signal from theoptical source 202. In certain embodiments, the optical signal includesa carrier wave, which the optical modulator 204 modulates based at leastin part on a reference signal. For instance, the optical modulator 204may include an electro-optic modulator configured to modulate anamplitude, phase, frequency, or polarization of the optical signal withan applied voltage or low-frequency electric field. In particular, theapplied voltage or low-frequency electric field of the electro-opticalmodulator causes directional forces which distort a position,orientation, characteristic, or shape, of a material through which theoptical signal is propagated. Controlled manipulation of the materialadjusts a refractive index, and modulates the optical signal asspecified by the reference signal.

As discussed in further detail below, after modulation by the opticalmodulator 204, the optical signal may be transmitted along thefiber-optic path 222 to the retro-reflector 210, retro-reflected by theretro-reflector 210, and received at the photodetector 212. Theretro-reflected optical signal may then be used to generate thereference signal used to drive the optical modulator 204. Such anarrangement supports continuous and self-sustained oscillations of theoptoelectronic oscillator system 200, and generation of a stable RFoutput.

In certain embodiments, the optical signal is transmitted along thefiber-optic path 222 from the optical modulator 204 to the opticalcirculator 206 before being received and retro-reflected at theretro-reflector 210. The optical circulator 206 is also positioned alongthe fiber-optic path 222 and configured to receive the optical signal,and direct the optical signal to the retro-reflector 210. In at leastone example, the optical circulator 206 is configured to direct theoptical signal based at least in part on a direction of propagation ofthe optical signal along the fiber-optic path 222. That is, the opticalcirculator 206 may be used to separate the optical signal received fromthe optical modulator 204 from a retro-reflection of the optical signalreceived from the retro-reflector 210.

In one example, the optical circulator 206 includes a first port inoptical communication with the optical modulator 204 along thefiber-optic path 222, a second port in optical communication with theretro-reflector 210 along the fiber-optic path 222, and a third port inoptical communication with the photodetector 212 along a feedback path.The feedback path is represented in FIG. 2 by arrow 218. Responsive toreceiving the optical signal from the optical modulator 204, the opticalcirculator 206 is configured to direct the optical signal along thefiber-optic path 222, and in a direction of the retro-reflector 210.While discussed in one example as including an optical circulator, invarious other embodiments the optical circulator 206 may include anyother suitable optical device, such as a high precision device foroptical rotation.

As FIG. 2 shows, after emission from the optical circulator 206 in thedirection of the retro-reflector 210, the optical signal may be receivedand propagate along a fiber link delay 208 interposed between theoptical circulator 206 and the retro-reflector 210. For example, thefiber link delay 208 may include a single optical fiber link. Inparticular, the fiber link delay 208 may have a length on the order ofabout 10 km. In various other embodiments, the fiber link delay 208 mayhave a length within a range of 1 m-100 km. In contrast to the doublepass experiments discussed above, which were tested on systems havingoptical fiber lengths of up to 1840 km, the significantly shorter fiberlink delays of the examples discussed herein allow a larger bandwidth ofactive stabilization, as well as inherent fiber noise correction basedon individual measurements. That is, as a result of the large roundtriptime (source to destination and back) of the experiment optical systems,the bandwidth of noise correction within those tested systems is limitedto noise on the order of tens or hundreds of Hertz (Hz). Such a limitedbandwidth of noise correction undesirably restricts the noise correctionperformance of the system, and in particular, makes noise correctionwithin the close-in phase noise region (i.e., frequencies within 100 kHzof the carrier wave frequency) challenging. The close-in phase noiseregion is principally important in numerous applications, such asadvanced radar. In contrast, because the fiber delay link 208 of variousembodiments has a much shorter length when compared to those of thetested systems, various embodiments permit noise correction beyond 10kHz.

As shown, the retro-reflector 210 is positioned at a second end of thefiber-optic path 222 to receive the optical signal from the fiber linkdelay 208. In various embodiments, the retro-reflector 210retro-reflects the received optical signal back along the fiber-opticpath 222 to cancel at least a frequency range of noise (e.g., inherentfiber noise) in the retro-reflected signal. That is, the retro-reflector210 may be positioned along the fiber-optic path 222 to reflect theoptical signal back in a direction of the optical source 202 withminimum scattering and substantially the same amplitude. While in oneembodiment, the retro-reflector 210 may include a Faraday mirror, invarious other embodiments, the retro-reflector 210 may include anysuitable passive retro-reflector configured to exactly reverse thepropagation direction of the optical signal. In other embodiments, theretro-reflector 210 may include a phase conjugate mirror. Accordingly,in certain embodiments, the retro-reflected optical signal includes aconjugate beam of the optical signal, the conjugate beam having theinherent fiber noise from the fiber-optic path removed. While in oneembodiment a particular frequency range of the inherent fiber noise maybe canceled, in other embodiments, the entire range of inherent fibernoise may be canceled.

While discussed above as including a passive retro-reflector, such as astandard mirror, in various other implementations, the retro-reflector210 shown in FIG. 2 may include an active retro-reflector. For example,an active retro-reflector may include a piezoelectric mounted opticaldevice that can be manipulated based on a control signal received fromother system components, such as the one or more electrical componentshown as component block 214. In particular, the active retro-reflectormay be systematically adjusted at a high frequency rate to cancel noisein the system 200 (e.g., environmental noise from fiber length changes).

While in one embodiment the retro-reflector 210 retro-reflects thereceived optical signal to directly cancel at least a particularfrequency range of inherent fiber noise, in various other embodimentsthe retro-reflected optical signal may be used by other systemcomponents to cancel all (or a particular frequency range) of theinherent fiber noise. In other embodiments, the retro-reflector 210 mayretro-reflect the optical signal to directly cancel a first frequencyrange of the inherent fiber noise, and the other system components mayuse the retro-reflected optical signal to cancel a remainder of theinherent fiber dependent, a second frequency range of the inherent fibernoise, or other noise within the system (e.g., environmentally inducednoise). For example, other noise cancelling operations may be performedby the one or more electrical component (represented by component block214) and/or the one or more optical component (represented by componentblock 216). Such embodiments are further discussed below.

The retro-reflected optical signal propagates through the fiber linkdelay 208 and along the fiber-optic path 222 in substantially anopposite direction of the propagating optical signal. Theretro-reflected optical signal is received by the optical circulator206, and directed along the feedback path to the first photodetector212, as discussed above. As FIG. 2 shows, in certain examples, thefeedback path includes an optical fiber coupled between the opticalcirculator 206 and the first photodetector 212, and an electricalconnection between the first photodetector 212 and the optical modulator204. In various embodiments, the optoelectronic oscillator system 200further includes an output coupled within the feedback path andconfigured to provide a stabilized RF output signal. The output isrepresented in FIG. 2 by arrow 220.

In various embodiments, the first photodetector 212 is positioned alongthe feedback path and receives the retro-reflected optical signal fromthe optical circulator 206. Responsive to receiving the retro-reflectedoptical signal, the first photodetector 212 generates the referencesignal to be provided to the optical modulator 204. The reference signalmay be used by the optical modulator 204 as a reference to stabilizegenerated microwave or millimeter wave (RF) signals modulated onto thefiber-optic path 222. For example, the optical modulator 204 may adjustan amplitude, phase, frequency, or polarization, of the optical signalbased on the reference signal. As FIG. 2 shows, in one example the firstphotodetector 212 includes a photo-diode configured to convert theretro-reflected optical signal into a corresponding current.

In certain embodiments the system 200 may include one or more electricalcomponent and/or one or more optical component, such as the one or moreelectrical component represented by the component block 214 and the oneor more optical component represented by component block 216. Electricalcomponents and optical components may include any of an amplifier, afilter, a phase shifter, an additional modulator, or any other elementsuitable for optimizing noise cancellation performance. In particular,the one or more electrical component and/or optical component may cancelinherent fiber noise in the optical signal based at least in part on theretro-reflected optical signal.

For example, in certain embodiments the optical modulator 204 may bedirectly coupled via a second fiber-optic path to the secondphotodetector 224, which may also include a photo-diode (as shown inFIG. 2). Used in conjunction with the reference signal generated by thefirst photodiode 212 (based on the retro-reflection of the opticalsignal), an output of the second photodiode 224 may be used to stabilizethe system 200, and cancel inherent fiber noise. In particular, anoutput of the second photodetector 224 may be coupled to the one or moreelectrical component represented by component block 214. Accordingly,the output from the second photodetector 224 may be received by the oneor more electrical component and combined and/or compared with thereference signal to generate a correction signal, or adjust thereference signal being provided to the optical modulator 204.

In a particular example, the one or more optical component includes anadditional modulator configured to receive the correction signal via aninterconnection between the additional modulator and the one or moreelectrical component. Responsive to receiving the correction signal, theadditional modulator modulates an amplitude, phase, frequency, orpolarization, of the retro-reflected optical signal to cancel at least afrequency range of the inherent fiber noise in the retro-reflectedoptical signal. For instance, the additional modulator may include anacousto-optic modulator. However, in various other embodiments theadditional modulator may cancel other noise in the system 200, such asat least a frequency range of environmentally induced noise. Whilediscussed herein as performed by the one or more optical component(represented by component block 216), in various other embodiments,similar processes may be performed by the one or more electricalcomponent (represented by component block 214) to cancel noise in thesystem 200. By such an arrangement, in-loop and out-of-loop correctionsmay be made to the amplitude and phase of the RF output signal generatedby the optoelectronic oscillator system 200.

Electrical components may be controlled or implemented by a controller,for example. The controller may include a single controller; however, invarious other embodiments the controller may consist of a plurality ofcontrollers and/or control subsystems, which may include an externaldevice, signal processing circuitry, or other control circuit. Inparticular, the controller may include analog processing circuitry(e.g., a microcontroller) and/or digital signal processing circuitry(e.g., a digital signal processor (DSP)). For instance, themicrocontroller of various embodiments may include a processor core,memory, and programmable input/output components.

FIG. 3 shows an example architecture for an alternately coupledoptoelectronic oscillator system 300 according to various aspects andembodiments. The optoelectronic oscillator system 300 of FIG. 3 mayinclude many of the same components as included within theoptoelectronic oscillator system 200 shown in FIG. 2. For example, afirst optical source 302, a first optical modulator 304, a first opticalcirculator 306, a first photodetector 312, a third photodetector 332,first electrical components (shown as component block 314), and firstoptical components (shown as component block 316), may be the same asthe optical source 202, the optical modulator 204, the opticalcirculator 206, the photodetectors 212, 224, and the one or more opticaland electrical components discussed above with reference to at leastFIG. 2, respectively. Similarly, a second optical source 324, a secondoptical modulator 318, a second optical circulator 310, a secondphotodetector 326, a fourth photodetector 334, second optical components(shown as component block 330), and second electrical components (shownas component block 328), may also be the same as the optical source 202,the optical modulator 204, the optical circulator 206, thephotodetectors 212, 224, and the one or more optical and electricalcomponents discussed above with reference to at least FIG. 2,respectively.

In various embodiments, the optoelectronic oscillator system 300includes two opposed optoelectronic oscillator devices coupled along thesame fiber-optic path 322. In certain embodiments, the twooptoelectronic oscillator devices generate counter-propagating opticalsignals to be used to cancel noise in the system 300, and constructivelyreinforce the desired RF output signal. In particular, the opticalsignal from one optoelectronic oscillator may be used to generatereference signals for the optical modulator of the other optoelectronicoscillator device. As discussed above, the reference signals may then beused to stabilize the RF output signal from that optoelectronicoscillator.

In certain embodiments, the first optical source 302 is positioned at afirst end of the fiber-optic path 322, and the second optical source 324is positioned at a second end of the fiber-optic path 322. Each opticalsource 302, 324 is optically coupled to a corresponding opticalmodulator (i.e., the first optical modulator 304 and the second opticalmodulator 318). The first optical source 302 is positioned to generateand transmit a first optical signal along the fiber-optic path 322 tothe first optical modulator 304. The first optical modulator 304 thenreceives the first optical signal, and modulates an amplitude, phase,frequency, or polarization, of the first optical signal, as discussedabove with reference to the optical modulator 204 shown in FIG. 2. Aftermodulation, the first optical signal is transmitted along thefiber-optic path 322 and directed by the first optical circulator 306 tothe second optical circulator 310. As shown, the first optical signalpropagates through the fiber link delay 308 interposed between the firstoptical circulator 306 and the second optical circulator 310 beforebeing received by the second optical circulator 310.

Once received at the second optical circulator 310, the first opticalsignal is directed by the second optical circulator 310 along a feedbackpath of the second optoelectronic oscillator device (i.e., a secondfeedback path). The first optical signal is then received by the secondphotodetector 326 and a second reference signal for the second opticalmodulator 318 is generated. The second optical modulator 318 isconfigured to receive the second reference signal from the secondphotodetector 326 (and second electrical components) and cancel at leasta frequency range of noise in a second optical signal received from thesecond optical source 324. For example, the second optical modulator 318may be configured to receive the second reference signal and amplitudemodulate the second optical signal to cancel the noise from the fiberlink delay 308 (e.g., inherent fiber noise). The second opticalmodulator 318 may then transmit the second optical signal along thefiber-optic path 322 in a reverse direction of the first optical signal.In the reverse direction, the propagation of the second optical signalis similar to that of the first optical signal. In various embodiments,the second optical circulator 310 is positioned to receive the secondoptical signal and direct the second optical signal to the first opticalcirculator 306.

After propagating through the fiber delay link 308, the second opticalsignal is received at the first optical circulator 306 and is directedto the first photodetector 312. The first photodetector 312 may then beconfigured to receive the second optical signal along the feedback pathof the first optoelectronic oscillator, and generate a first referencesignal to be provided to the first optical modulator 304 after passingthrough the first electrical components. In various embodiments, thefirst and second reference signals may include RF output signals to beprovided to the outputs (shown as arrows 320 and 321) of the first andsecond optoelectronic oscillator devices. Accordingly, in variousembodiments the second optical signal from the second optoelectronicoscillator may be used to correct for noise in the first optical signal,and may be used to drive the first optoelectronic oscillator. Whiledescribed herein with reference to a single direction, in various otherembodiments, such processes may be performed in the reverse direction,or progressively in both directions along the fiber-optic path 322between the two optoelectronic oscillators.

In certain examples, the electrical components or optical components ofthe first and second optoelectronic oscillator devices may also cancelnoise in the optical system 300 based at least in part on the firstoptical signal and/or the second optical signal. In one example, thefirst optical modulator 304 is directly coupled to the thirdphotodetector 332, which may include a photo-diode (as shown in FIG. 3).The second optical modulator 318 may be similarly coupled to the fourthphotodetector 334. Used in conjunction with the corresponding firstreference signal generated by first photodetector 312, an output of thethird photodetector 332 may be used to stabilize the system 300 andcancel noise, such as inherent fiber noise from the fiber link delay308. In particular, an output of the third photodetector 332 may becoupled to the first electrical components (represented by componentblock 314). The output from the third photodetector 332 may be receivedby the first electrical components and combined and/or compared with thefirst reference signal to generate a correction signal, or adjust thefirst reference signal provided to the first optical modulator 304.

In one example, first optical components include an additional modulatorconfigured to receive the correction signal via an interconnectionbetween the additional modulator and the first electrical components.Responsive to receiving the correction signal, the additional modulatormodulates an amplitude, phase, frequency, or polarization, of the secondoptical signal to cancel at least a frequency range of inherent fibernoise in the second optical signal. However, in various otherembodiments the additional modulator may cancel other noise in thesystem 300, such as a least a frequency range of environmentally inducednoise. While discussed herein as performed by the first opticalcomponents (represented by component block 316), in various otherembodiments similar processes may be performed by the first electricalcomponents (represented by component block 314). In various embodiments,the fourth photodetector 334, second optical components (represented bycomponent block 330), and the second electrical components (representedby component block 328), and perform similar processes in the reversedirection.

As discussed above with reference to FIG. 2 and FIG. 3, severalembodiments perform processes for reducing noise in optoelectronicoscillators. In particular, several embodiments perform processes foractive or passive cancellation of noise, for example inherent fibernoise. In some embodiments, these processes are executed by anoptoelectronic oscillator system, such as the system 200 described withreference to FIG. 2. One example of such a process is illustrated in theprocess flow shown in FIG. 4. According to this example, the process 400may include the acts of transmitting an optical signal, modulating theoptical signal, receiving and retro-reflecting the optical signal,directing the retro-reflected optical signal by an optical circulator,and providing an output signal.

In act 402, the process 400 may include transmitting an optical signalalong the fiber-optic path from an optical source positioned at thefirst end of the fiber-optic path. While in one embodiment, the process400 may further include generating a continuous optical signal with awavelength within the spectrum of visible light, in various otherembodiments, the optical source may generate an optical signal of anyother suitable wavelength of electromagnetic radiation. In response totransmitting the optical signal, the optical signal may be received atan optical modulator.

In various embodiments, act 404 includes modulating the received opticalsignal at the optical modulator based at least in part on a referencesignal. As discussed with reference to the optical modulator 204 shownin FIG. 2, modulating the optical signal may include adjusting anamplitude, phase, frequency, or polarization, of the optical signal. Inparticular, the optical modulator may adjust one or more of theseproperties of the optical signal to stabilize and sustain an RFelectro-optic oscillation.

Following modulation by the optical modulator, in act 406 the process400 includes receiving and retro-reflecting the optical signal at aretro-reflector positioned at a second end of the fiber-optic path. Incertain embodiments, the process 400 includes transmitting the opticalsignal along the fiber-optic path from the optical modulator to theoptical circulator, before receiving the optical signal at theretro-reflector. As discussed above with reference to the opticalcirculator 206 shown in FIG. 2, the optical circulator may be positionedalong the fiber-optic path and may perform processes for directing theoptical signal based at least in part on a direction of propagation ofthe optical signal along the fiber-optic path. That is, the opticalcirculator may separate the optical signal received from an opticalmodulator from the retro-reflected of the optical signal received fromthe retro-reflector.

In certain embodiments, the process 400 may also include the act ofpropagating the optical signal along the fiber link delay beforereceiving the optical signal at the retro-reflector. Similarly, theprocess 400 may include the act of propagating the retro-reflectedoptical signal along the fiber link delay responsive to retro-reflectingthe optical signal. While the length of the fiber link delay may bechosen based on the particular application of the optoelectronicoscillator system, as discussed above, in various embodiments the fiberlink delay has a length on the order of 10 km, or within a range of 1m-100 km.

Returning to act 406, in various embodiments the process 400 includesretro-reflecting the received optical signal back along the fiber-opticpath and canceling at least a frequency range of inherent noise in theretro-reflected signal. That is, the process 400 may includeretro-reflecting the optical signal back in a direction of the opticalsource with minimum scattering and substantially the same amplitude.While in one embodiment, the retro-reflector may include a Faradaymirror, in various other embodiments, the retro-reflector may includeany suitable passive retro-reflector configured to exactly reverse thepropagation direction of the optical signal. For example, theretro-reflector may include a phase conjugate mirror. Accordingly, incertain embodiments, retro-reflecting the optical signal may includereflecting a conjugate beam of the optical signal, the conjugate beamhaving inherent fiber noise from the fiber-optic path canceled. While inone embodiment a particular frequency range of inherent fiber noise maybe canceled, in other embodiments, the entire range of inherent fibernoise may be canceled.

In act 408, the process 400 may include directing, with the opticalcirculator positioned along the fiber-optic path, the retro-reflectedoptical signal along a feedback path to a photodetector to generate thereference signal. In act 410, the process 400 may then optionallyinclude providing a stabilized RF output signal, which may include, forexample, the reference signal. As discussed herein, the reference signalmay be used by the optical modulator to adjust the properties (i.e.,phase, frequency, amplitude, or polarization) of the optical signal.

In certain other embodiments, the process 400 may further include theacts of receiving the optical signal at a second photodetector (e.g.,the second photodetector 224 shown in FIG. 2), and generating a controlsignal to cancel inherent fiber noise in the retro-reflected opticalsignal, or other noise within the system. That is, while in one exampleretro-reflecting the optical signal may include canceling at least afrequency range of inherent fiber noise in the retro-reflected opticalsignal, in other examples other components of the system may cancelinherent fiber noise. For example, the process 400 may include receivingan output of the second photodetector at the one or more electricalcomponents, and generating the control signal based on a combination ora comparison of the output from the second photodetector and thereference signal. In certain other embodiments, such processes mayinclude adjusting the reference signal based on the combination orcomparison.

In a particular example, the one or more optical component includes anadditional modulator and the process includes modulating theretro-reflected optical signal to cancel inherent fiber noise responsiveto receiving the control signal at the one or more optical component.While in one embodiment, modulating the retro-reflected optical signalmay include modulating an amplitude of the retro-reflected opticalsignal, in various other embodiments, the process 400 may include theact of modulating a phase, frequency, or polarization of theretro-reflected optical signal at the one or more optical component.While discussed herein as performed by the one or more opticalcomponent, in various other embodiments the process 400 may includesimilar acts performed by the one or more electrical component.

Accordingly, aspects and embodiments are directed to systems forreducing noise in optoelectronic oscillators, and methods of using thesame. Certain examples involve the implementation of a double passconfiguration optoelectronic oscillator system, which may includeoptical circulators, retro-reflectors, couplers, or other similarlyfunctioning elements, in the fiber-optic path. These examples allow forthe independent and individual correction and cancelation of effectsfrom inherent fiber dependent noise and environmentally induced fiberlength fluctuation through active, or passive, approaches. Additionalimprovements may also be achieved as a result of the proximity ofcomponents of the optoelectronic oscillator system of variousembodiments. Because such embodiments are compact and do not requiregeographically dispersed components, embodiments may be environmentallystabilized in a compact package.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. An optoelectronic oscillator system comprising:an optical source positioned at a first end of a fiber-optic path, theoptical source being configured to transmit an optical signal along thefiber-optic path; an optical modulator positioned to receive the opticalsignal from the optical source and modulate the optical signal based onat least a received reference signal; a retro-reflector positioned at asecond end of the fiber-optic path, the retro-reflector being configuredto receive and retro-reflect the optical signal along the fiber-opticpath, the retro-reflected optical signal having at least a frequencyrange of inherent fiber noise canceled; and an optical circulatorpositioned along the fiber-optic path and interposed between the opticalmodulator and the retro-reflector, the optical circulator beingconfigured to direct the optical signal to the retro-reflector anddirect the retro-reflected optical signal along a feedback path to afirst photodetector to generate the reference signal.
 2. Theoptoelectronic oscillator system according to claim 1, furthercomprising an optical component positioned along the fiber-optic pathand interposed between the retro-reflector and the optical circulator,and an electrical component positioned along the feedback path andinterposed between the first photodetector and the optical modulator,wherein at least one of the optical component and the electricalcomponent includes one of an amplifier, a filter, an additionalmodulator, and a phase shifter.
 3. The optoelectronic oscillator systemaccording to claim 2, further comprising a second photodetectorinterposed between the optical modulator and the electrical component.4. The optoelectronic oscillator system according to claim 3, whereinthe electrical component is configured to receive an output from thesecond photodetector, receive the reference signal from the firstphotodetector, and transmit a control signal to the optical component tocancel the frequency range of inherent fiber noise in theretro-reflected optical signal.
 5. The optoelectronic oscillator systemaccording to claim 2, further comprising a fiber link delay positionedalong the fiber-optic path and interposed between the retro-reflectorand the optical component, wherein the fiber link delay has a length ofsubstantially 10 km.
 6. The optoelectronic oscillator system accordingto claim 5, wherein the fiber link delay includes a single optical fiberlink.
 7. The optoelectronic oscillator system according to claim 1,wherein the retro-reflector is configured cancel the frequency range ofinherent fiber noise by retro-reflecting the optical signal.
 8. Theoptoelectronic oscillator system according to claim 7, wherein theretro-reflector includes a phase conjugate mirror.
 9. An optoelectronicoscillator system comprising: a first optical source positioned at afirst end of a fiber-optic path and optically coupled to a first opticalmodulator, the first optical source being configured to transmit a firstoptical signal along the fiber-optic path; a second optical sourcepositioned at a second end of the fiber-optic path and optically coupledto a second optical modulator, the second optical source beingconfigured to transmit a second optical signal along the fiber-opticpath, and the second modulator being configured to cancel at least afirst frequency range of noise in the second optical signal; a fiberlink delay positioned along the fiber-optic path and interposed betweenthe first optical modulator and the second optical modulator; and afirst optical circulator positioned along the fiber-optic path andinterposed between the first optical modulator and the fiber link delay,the first optical circulator being configured to receive the secondoptical signal from the second optical modulator and direct the secondoptical signal along a first feedback path to a first photodetector togenerate a first reference signal.
 10. The optoelectronic oscillatorsystem according to claim 9, further comprising: a second opticalcirculator positioned along the fiber-optic path and interposed betweenthe second optical modulator and the fiber link delay, the secondoptical circulator being configured to receive the first optical signalfrom the first optical modulator and the fiber link delay and direct thefirst optical signal along a second feedback path to a secondphotodetector to provide a second reference signal.
 11. Theoptoelectronic oscillator system according to claim 10, furthercomprising: a first optical component positioned along the fiber-opticpath and interposed between the first optical circulator and the fiberlink delay; and a second optical component positioned along thefiber-optic path and interposed between the second optical circulatorand the fiber link delay.
 12. The optoelectronic oscillator systemaccording to claim 11, further comprising: a first electrical componentpositioned along the first feedback path and interposed between thefirst photodetector and the first optical modulator; and a secondelectrical component positioned along the second feedback path andinterposed between the second photodetector and the second opticalmodulator, wherein at least one of the first electrical component, thesecond electrical component, the first optical component, and the secondoptical component, includes one of an amplifier, a filter, an additionalmodulator, and a phase shifter.
 13. The optoelectronic oscillator systemaccording to claim 12, further comprising: a third photodetectorinterposed between the first optical modulator and the first electricalcomponent; and a fourth photodetector interposed between the secondoptical modulator and the second electrical component.
 14. Theoptoelectronic oscillator system according to claim 13, wherein thefirst electrical component is configured to receive an output from thethird photodetector, receive the first reference signal from the firstphotodetector, and transmit a control signal to the first opticalcirculator to cancel at least a second frequency range of the noise inthe second optical signal.
 15. The optoelectronic oscillator systemaccording to claim 13, wherein the fiber link delay includes a singleoptical fiber link having a length of substantially 10 km.
 16. Theoptoelectronic oscillator system according to claim 9, wherein thesecond optical modulator is configured to receive the second referencesignal and amplitude modulate the second optical signal.
 17. Anoptoelectronic oscillator noise reduction method, the method comprising:transmitting an optical signal from an optical source positioned at afirst end of a fiber-optic path along the fiber-optic path; modulatingthe optical signal with an optical modulator positioned to receive theoptical signal along the fiber-optic path based on at least a referencesignal; receiving and retro-reflecting the optical signal at aretro-reflector positioned at a second end of the fiber-optic path, theretro-reflected optical signal having at least a frequency range ofinherent fiber noise canceled; and directing, with an optical circulatorpositioned along the fiber-optic path, the retro-reflected opticalsignal along a feedback path to a photodetector to generate thereference signal.
 18. The method according to claim 17, whereinretro-reflecting the optical signal includes canceling the frequencyrange of inherent fiber noise in the retro-reflected optical signal. 19.The method according to claim 18, wherein the retro-reflector includes aphase conjugate mirror, and wherein retro-reflecting the optical signalincludes reversing a propagation direction of the optical signal. 20.The method according to claim 17, further comprising: receiving theoptical signal at a second photodetector interposed between the opticalmodulator and an electrical component positioned in the feedback path;receiving the reference signal and an output of the second photodetectorat the electrical component; and transmitting a control signal from theelectrical component to an optical component positioned along thefiber-optic path to cancel the frequency range of inherent fiber noisein the retro-reflected optical signal.